public class code_1 {
    //递归
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if(list1==null) return list2;
        if(list2==null) return list1;
        if(list1.val<list2.val) {
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;
        } else {
            list2.next = mergeTwoLists(list1,list2.next);
            return list2;
        }
    }
}
